In most domestic hot water heating systems, an oil or gas fired burner maintains a water supply between predetermined temperature limits. This supply may be either a reserve tank or a coil in the furnace or boiler. For example, a typical domestic hot water heating system will turn on the burner when the temperature of the reserve water decreases to a predetermined temperature, and will turn off the burner when the temperature of the reserve water increases to another predetermined value. The two temperature limits are set in a controller generally known as an aquastat.
This operation serves to maintain a domestic hot water reserve within given temperature limits even though at many hours of the day there is no demand for domestic hot water. Presently known set-back controls for water heaters are based on predetermined hours of the day where statistics tend to show there is less demand for domestic hot water. However, during the set-back periods, there may be a time when there is a demand for hot water which cannot be supplied and the user must wait for expiration of the set-back period.
Accordingly, the present invention provides a new and improved control system which almost immediately senses a demand for domestic hot water and will, in the set-back mode, allow almost immediate burner operation to satisfy demand, even though the reserve water temperature may not be within the limits set on the aquastat.